This invention relates to electronic ballasts for gas discharge lamps and, in particular, to an electronic ballast having a microprocessor for managing the switching frequencies of the boost circuit and the inverter.
A fluorescent lamp is an evacuated glass tube with a small amount of mercury in the tube. The tube is lined with an adherent layer of a mixture of phosphors. Some of the mercury vaporizes at the low pressure within the tube and a filament in each end of the tube is heated to emit electrons into the tube, ionizing the gas. A high voltage between the filaments causes the mercury ions to conduct current, producing a glow discharge that emits ultraviolet light. The ultraviolet light is absorbed by the phosphors and re-emitted as visible light. "Instant start" lamps do not have heated filaments but rely on a very high starting voltage to initiate a discharge between specially designed electrodes in each end of the lamp.
A gas discharge lamp, such as a fluorescent lamp, is a non-linear load to a power line, i.e. the current through the lamp is not directly proportional to the voltage across the lamp. Current through the lamp is zero until a minimum voltage is reached, then the lamp begins to conduct. Once the lamp conducts, the current will increase rapidly unless there is a ballast in series with the lamp to limit current.
A resistor can be used as a ballast but a resistor consumes power, thereby decreasing efficiency, measured in lumens per watt. A "magnetic" ballast is an inductor in series with the lamp and is more efficient than a resistor but is physically large and heavy. A large inductor is required because impedance is a function of frequency and power lines operate at low frequency (50-60 Hz.)
An electronic ballast typically includes a rectifier for changing the alternating current (AC) from a power line to direct current (DC) and an inverter for changing the direct current to alternating current at high frequency, typically 25-60 kHz. Electronic ballasts also include a boost circuit between the rectifier and the inverter. As used herein, a "boost" circuit is a circuit that increases the DC voltage, e.g. from approximately 180 volts (assuming a 120 volt input) to 300 volts or more, for operating a lamp and for providing power factor correction. "Power factor" is a figure of merit indicating whether or not a load in an AC circuit is equivalent to a pure resistance, i.e. indicating whether or not the voltage and current are in phase. It is preferred that the load be the equivalent of a pure resistance (a power factor equal to one).
The boost circuit and the inverter each include switching transistors that operate at various frequencies during normal operation of a ballast. The boost circuit changes frequency with the line voltage to produce high power factor. The inverter typically includes a series resonant, direct coupled output, which means that the output voltage and the output current can be adjusted by changing frequency.
The frequency of the inverter at any given moment is relatively constant during normal operation, due, in part, to the use of ceramic resonators in microprocessor controlled ballasts, and therein lies a problem. A plurality of such ballasts will operate on very nearly the same frequency under normal operating conditions. The problem is that the frequencies are very nearly the same, but not exactly the same. When the frequencies are not exactly the same, the ballasts interfere with each other.
A linear lamp, such as a four foot long T8 fluorescent lamp, has a considerable distributed capacitance between the discharge within the tube and a fixture in which the lamp is mounted, which acts as a ground plane. The capacitance enables part of the lamp current to flow through the lamp tube into the ground plane and back to the ballast. The wires connecting a lamp to a ballast may provide additional capacitive coupling. When two or more ballasts are connected to lamps that are close to each other, the current from one ballast can return through the other ballast and vice-versa. When the inverters in the ballasts are operating at nearly the same frequency, e.g. within 1-50 Hz of each other, visible beat frequencies (flicker) can occur. The flicker is at least annoying and could be interpreted as incipient lamp failure or incipient ballast failure, leading to pointless replacement and frustration.
Another kind of interference is electromagnetic interference (EMI). The frequency of the boost circuit is continuously changing in normal operation but becomes constant if the applied voltage is direct current, as it is in some emergency lighting configurations and in areas supplied by direct current. Although the input to a ballast is filtered to reduce (EMI), some of the high frequency signal from the boost circuit will pass through the filter. The filter typically used with an electronic ballast is more than adequate if the boost circuit is constantly changing frequency because the EMI is spread over many frequencies. If the ballast is operated on DC, the EMI is concentrated at a single frequency and the ballast may not comply with governmental or quasi-governmental specifications.
It is known in the art to provide self-dimming when the input voltage is reduced, as in a brownout by a power company. "Self-dimming" is a reduced power output solely in response to a reduced line voltage, without a separate control line or control signal to the ballast. It is also known in the art to provide a universal input voltage (110-277 volts, DC/50-60 Hz.). A problem with a device providing a universal input is that the device automatically draws more current when input voltage is reduced, which interferes with the power company's ability to control power distribution. It is desired to have both universal input voltage and self-dimming.
In general, the problem is one of managing the switching frequencies within an electronic ballast, as opposed to simply letting the ballast determine its own operating points. Frequency management is not what is disclosed in U.S. Pat. No. 5,680,015 (Bernitz et al.), which describes a ballast for a high intensity discharge (HID) lamp wherein a microprocessor controls the frequency of the inverter to avoid mechanical resonances within the HID lamp. Specifically, the Bernitz et al. patent discloses testing the lamp for viable operating frequencies within a range of frequencies. A list of viable operating frequencies is stored in memory. If mechanical resonance is detected, causing the lamp to flicker, the microprocessor changes to another of the frequencies from the list. Thus, the ballast described in the Bernitz et al. patent is optimizing operation of the lamp itself, an intrinsic purpose.
Frequency management also arises, for example, in response to abrupt changes in condition. When a boost converter provides power factor correction, it is necessary that the frequency response of the converter be less than twice the line frequency. Low frequency response leads to very slow response to transient conditions, e.g. lamp removal. If a boost is running at full power and is suddenly unloaded, the output from the boost can well exceed maximum voltage ratings of components within the ballast. When the boost responds to the voltage spike and shuts off, it may remain shut off for so long that it shuts off power for the control circuitry, including a microprocessor controlling the ballast. If the microprocessor turns off and turns on, it begins a cold start sequence that may be inappropriate for the conditions, e.g. parameter tables may be reset to default values. It is desired to provide a controlled response to abrupt changes in condition that does not turn off the microprocessor.
In view of the foregoing, it is therefore an object of the invention to provide an electronic ballast that manages the switching frequencies in an electronic ballast for extrinsic purposes.
A further object of the invention is to provide an electronic ballast that manages the frequency of the boost circuit for reduced EMI.
Another object of the invention is to provide an electronic ballast that manages the frequency of the inverter for reduced interference with other ballasts.
A further object of the invention is to provide an electronic ballast that minimizes coupling to other ballasts.
Another object of the invention is to provide an electronic ballast that shifts the frequency of operation of an electronic ballast to reduce or eliminate interference.
A further object of the invention is to provide a ballast with both universal input voltage and self-dimming (brown-out) capability.
Another object of the invention is to provide a boost powered microprocessor that can respond to abrupt changes in condition without turning off.